Among the equipment and fittings usually found on the wellhead of an oil or gas well, there is a valve called the master valve. This valve will open or close the passage of fluids and gases in the well to any related equipment or piping above it, and is the last valve in a line of valves which control the flow of fluids or gases from the well. This valve develops leaks or otherwise becomes unserviceable over its lifetime and must be taken off to be repaired. The fluid or gases that are under pressure in the well must be plugged off in order to do this. For the explanations and descriptions which follow, we will use the word gas to mean both gases and fluids which may be in the well, and the word tubing to mean both tubing and casing. There are several ways to plug off the well, each with its own drawbacks. As examples, three such methods will be described. The first way is to `kill` the well by pumping a fluid down the well, which fluid is of such a density that it will counterbalance the pressure from the producing formation and create a zero pressure at the master valve. The valve can then be removed. There is a risk in this method, as a well that has been `killed` may not always come back into production when the counterbalancing fluid has been removed. A second method is to install a packer in the tubing. To do this, a mobile service rig is driven over the wellhead, and a packer run into the well on a length of steel pipe, through appropriate sealing mechanisms above the master valve, and set in place by rotating and loading weight or pulling up. The steel pipe is then unlatched from the packer, taken out of the well and the master valve may be removed. When the master valve or other equipment has bee replaced, the service rig runs in the steel pipe again, latches onto the packer, undoes the packer by rotating, loading weight or pulling, and removes it from the well. The disadvantages of this method are mainly financial, as the service rig is a large piece of mobile equipment which takes several people to run. A third method is to freeze off the wellhead. In this method, a heavy gel, about the consistency of tooth paste, is pumped through the master valve into the tubing for a short length below the master valve. Then around the outside of this area of the tubing, under appropriate insulation, liquid nitrogen is circulated in a coil and the resulting cooling freezes the gel in the tubing, thus effectively plugging the well. The master valve can then be removed and replaced. This method has its own inherent problems and dangers due to the number of variables encountered.